The present invention relates to an ignition timing control system for restraining the knocking of an internal combustion engine, and more particularly to a system for detecting the malfunction of a vibratory acceleration sensor which is used to detect the knocking.
It is desired that the ignition timing of an internal combustion engine be set so as to be most efficient for the running mode of the engine. It is also generally desired that the ignition timing be so set as to fall within a range, in which the engine is free from any knocking, as close as possible to the "MBT" (i.e., Minimum advance for Best Torque). However, most of the ingition timing control systems according to the prior art are of such mechanical type that their ignition angle advancing characteristics are unstable due to variation in materials and due to aging. In order to prevent knocking, therefore, the actual ignition timing is set at a considerably retarded angle with respect to the aforementioned desirable ignition angle advancing characteristics.
At this retarded angle, the efficiency of the engine is degraded. Moreover, even if an ignition timing control system is used having neither significant materials variation nor aging problems, the knocking phenomena are influenced by the temperature and moisture of the intake air as well as the air-fuel ratio of the engine. As a result, even if the ignition timing is so set that no knocking takes place under a certain condition, there is a danger that knocking will nevertheless occur under different running conditions.
If, with this in mind, a control function is conducted to detect the knocking and retard the ignition timing if knocking occurs, the ignition timing can be so adjusted that little knocking takes place even if an error is caused in the ignition angle advancing characteristics due to the material characteristic variations inherent in the mechanical type or due to the difference in running conditions, as has been described hereinbefore.
Thus, the ignition timing control system of the above type is able to achieve the optimum ignition timing by setting in advance the reference ignition angle advancing characteristics at a more advanced angle so as to come closer to the MBT and by effecting an angle retarding control at a proper time in response to the occurrence of knocking.
In case, however, the knocking detecting system malfunctions, there arises a problem. Specifically, if the knocking detecting system malfunctions, no knocking information is obtained so that the proper angle retarding control cannot be accomplished. This results in a serious problem that the engine is so highly vibrated by the knocking that it may possibly be damaged.
In order to cope with this problem, it is desired to control the ignition timing such that no knocking takes place, or to generate a warning signal.